In today's business and scientific world, color is essential as a component of communication. Color facilitates the sharing of knowledge, and as a result companies involved in the development of digital color print engines are continuously seeking to improve the image quality of such products. One of the elements that affect image quality is the ability to consistently produce the same image quality on a printer over time. Colors on a printer tend to drift due to ink/toner variations, temperature fluctuations, type of media used, environment, etc. There has been a long felt commercial need for efficiently maintaining print color predictability, particularly as print media places greater importance on the accurate representation of merchandise in print and display media.
Almost all color printers present some amount of temporal color drift, regardless of the nature of the print engine, etc. To maintain consistent color reproduction, it is generally necessary to monitor the printer performance and from time to time apply corresponding color adjustment to the printer. Although a full color characterization, for example as described by R. Bala, in “Device Characterization”, Digital Color Imaging Handbook, Chapter 5 (CRC Press, ©2003), certainly can correct temporal color drift, some simpler color correction methods based on the 1-D tonal response curve (TRC) calibration for each individual channel usually are sufficient and are much easier to implement. The 1-D approach also readily lends itself to the use of in-line sensors for color measurement as described, for example in U.S. Pat. No. 6,744,531 to L. Mestha et al. Many different apparatus and methods have been proposed on this subject and most of them, like most color calibration and color characterization methods (see e.g., “Device Characterization” by R. Bala, supra), are halftone dependent.
The most popular technique to build a printer characterization transform involves printing and measuring a large set of color samples, i.e. CMY(K) patches, in conjunction with mathematical fitting and interpolation to derive CMY(K)->Lab mappings. The accuracy of the characterization transform clearly depends on the number (N) of patches printed and measured. Crucially, note that these patches correspond to continuous tone CMY digital values, i.e. their binary representation is halftone dependent. Therefore, for color printers equipped with multiple halftone screens or halftone methods, the measurement and the correction has to be repeated as many times as the number of the halftones. For example, for a CMYK color printer equipped with five different halftone screens, monitoring the four channel TRCs needs a total of 4×5×N patches, where N is the number of chosen digital levels to print and to measure for each channel and each halftone screen. From practice it has been shown that the number of patches N for each halftone cannot be too small because the halftone TRC is usually not a smooth curve. The shape of a TRC depends not only on the design of the digital halftone screen, but also on the dot overlapping and other microscopic geometries of the physical output from the printer.
For example, FIG. 1 illustrates the measured TRCs for a 200 line per inch (lpi) halftone screen. The solid and the dashed lines represent the results from printouts on two different paper substrates, respectively. It should be noted that neither TRC is very smooth; instead, they have a piecewise nature. If there are not enough levels printed and measured, an accurate estimate of the true TRC for the full range (i.e. digital level 0-255) cannot be derived even using sophisticated sampling and interpolation methods. It is not unusual, therefore, to find that even N=16 patches (for each colorant channel) are not enough for a good TRC estimation. Thus, existing color correction methods when applied across multiple halftone screens likely prove to be time and measurement intensive and may not be desirable especially for an in-line correction.
In accordance with aspects of the disclosure temporal color drift of a printer can be corrected by a re-calibration of the color printer, quite often, by re-measuring and obtaining the tone response curves (TRCs) of the individual channels. For a printer equipped with multiple halftone screens or halftone methods, the color calibration process has to be repeated for each halftone selection. Furthermore, due to dot overlapping and other intrinsic microscopic structure, the native tone response of a halftone screen is seldom a smooth function. To get an accurate calibration of a halftone screen, the required number of patches to print, or the number of digital levels, cannot be too small. Therefore, it is believed beneficial to develop a halftone independent color drift correction method.
A halftone independent color correction method was proposed by Shen-ge Wang, in co-pending U.S. Application 11/343,656 for “Halftone Independent Color Drift Correction” filed Jan. 31, 2006 (US2007/0177231-A1), hereby incorporated by reference in its entirety, based on a 2×2 binary printer model for printers with isomorphic resolution up to 600 dpi.
Halftone independent color correction methods disclosed in above references are based on a 2×2 binary printer model for printers with isomorphic resolution up to 600 dpi. A fundamental assumption of the 2×2 printer model is that the rendered physical spot is no more than two logical image pixels wide. However, hi-addressability xerographic printers with printing resolutions much higher than 600 dpi violate this assumption. A halftone-independent color correction scheme was developed based on combining color predictions made using the 2×2 printer model for targets of varying resolution. Experiments conducted using two different 4800×600 hi-addressability printers confirm that the proposed color correction is very good and comparable to measurement and computation intensive halftone-dependent methods. Further benefits lie in the computational simplicity of the proposed scheme, and patch measurements that may be acquired by either a colorimetric device or a common desktop scanner.
Disclosed in embodiments herein is a model-based, halftone independent method for characterizing a high-addressability printer equipped with a plurality of halftone screens, comprising: printing a target set of basic patches, said target set including patches having at least two resolutions and comprised of a fundamental binary pattern independent of a halftone screen; measuring printer response from the target set; modeling a halftone independent characterization of the printer with a mathematical transformation using the measured response; modeling a first halftone dependent characterization of the printer with the mathematical transformation to generate a first predicted result using a halftone screen; comparing a measured response of the printer using this halftone screen with the predicted result to define a correction factor corresponding to the halftone screen; and modeling a second halftone dependent characterization of the printer using a predicted response of the fundamental binary pattern and the correction factor.
Further disclosed in embodiments herein is a halftone independent method for high-addressability device characterization comprising: calculating a binary printer model which is halftone independent; retrieving one of a set of user-selected halftones and a corresponding halftone correction factor; deriving a halftone correction factor as a mathematical transformation between a true color value as measured from the device, for at least two resolutions, and the predicted color value; processing a device color value using the selected halftone, the online binary printer model and a halftone correction factor to predict colorimetric values; and using the device color value and the predicted colorimetric values to produce an improved printer characterization for the user selected halftone.
Also disclosed herein is a high-addressability printing system equipped with a plurality of halftone screens, comprising: memory storing at least one set of basic patches, said set including patches having at least two resolutions; a marking system for printing a target set of basic patches, said target set including patches having at least two resolutions and comprised of a fundamental binary pattern independent of a halftone screen; a colorimetric device, said device measuring printer response from the target set; and a color balance controller, said controller using the measured response and modeling a halftone independent characterization of the printer with a mathematical transformation to generate a first predicted result using a halftone screen, said color balance controller, further comparing a measured response of the printer using this halftone screen with the predicted result to define a correction factor corresponding to the halftone screen, and modeling a second halftone dependent characterization of the printer using a predicted response of the fundamental binary pattern and the correction factor.
The various embodiments described herein are not intended to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims.